Vaccination Timeliness Among US Children Aged 0-19 Months, National Immunization Survey–Child 2011-2021

Key Points Question Have there been changes in timeliness of vaccine receipt among US children aged 0 to 19 months, and did changes in on-time vaccination differ by socioeconomic indicators? Findings In this cross-sectional study of 2011 to 2021 National Immunization Survey–Child data for 179 154 children, receipt of the combined 7-vaccine series by recommended ages increased from 22.5% in 2011 to 35.6% in 2021. However, mean annual rates of improvement in on-time vaccination were lower for children from lower-income families and for children without private health insurance. Meaning These findings suggest that vaccination timeliness has improved overall, but widening disparities by socioeconomic status indicate continued barriers to accessing routine and timely vaccinations.


Determining age in days for parameters
To date, the main data sources used to measure days undervaccinated have been National Immunization Survey-Child (NIS-Child), electronic health record (EHR), and state and jurisdictional immunization information system (IIS) data. 7While EHR and IIS records may include birth and vaccination dates, these variables may be deidentified for research purposes by calculating age in days for each vaccination and then removing dates.Public use NIS-Child data files only contain children's age in days for each vaccine received; dates are not available.Therefore, we sought to determine parameters for measuring days undervaccinated using age-in-days variables.
In both Advisory Committee for Immunization Practices (ACIP) and Clinical Decision Support for Immunizations (CDSi) documentation, the minimum acceptable age of vaccine receipt is provided in weeks in age, with the exception of measles-mumpsrubella (MMR), varicella, and Hepatitis A (HepA) vaccines, for which 12 months is provided as the minimum age.Similarly, in both ACIP and CDSi documentation, the recommended interval between doses of multi-dose vaccinations is provided in weeks, with the exception the recommended interval between the 3 rd and 4 th doses of DTaP, which is provided in months.To calculate the minimum age of receipt and minimum acceptable interval between doses in days, ages or intervals provided in weeks were multiplied by 7 days.As allowable per ACIP recommendations, a 4-day grace period was subtracted. 8Similarly, to calculate the minimum age in days for MMR, varicella, and HepA vaccines, and the minimum interval between the 3 rd and 4 th doses of DTaP vaccines, we determined the minimum number of days necessary to achieve the minimum age or interval (which was provided in months), and subtracted 4 days.
The ACIP schedule provides a one-month or multiple-month recommended interval for when each vaccine dose should be given.Once that interval has elapsed, a child is considered late in receiving the vaccine.2][3][4][5][6] However, when the month and year a child was born is not available in the source data, there may be variability in this calculation as there is a range of possible ages in days.For example, the first dose of PCV is recommended at age 2 months.The day of birth is considered day 0. Since this first PCV dose is considered late the day the child turns 3 months old, the child could be as young as age 89 days if born in February or as old as age 92 days if born in July.To be consistent across milestone ages and avoid misclassifying children as undervaccinated, we used the oldest possible age in days a child could be to initiate the days undervaccinated count.To achieve this consistency, we assumed the child was born on July 1 st and the first year of life included a leap year, such that the following February (when the child turns age 7 months) has 29 days.The months of July and August both have 31 days.Similarly, December and January each have 31 days, so this calculation yields the oldest possible age in days a child could be and have received recommended vaccine doses on-time.

Updates to ACIP documentation
In review of the 2011-2022 ACIP schedules and accompanying documentation, we identified two changes that affected measurement of days undervaccinated.
Starting with the 2012 schedule, guidance was added specifying that the minimum acceptable interval between the 1 st and 3 rd HepB doses is 16 weeks, which we interpreted as 108 days (16 weeks x 7 days=112 days-4-day grace period=108 days).Of the nine routine childhood vaccine series we examined in this project, hepatitis B (HepB) is the only one where the ACIP schedule specified minimal intervals between non-subsequent doses.Since this documentation was added as clarification early in the study period, we applied the minimum interval parameter of 108 days between the 1 st and 3 rd HepB doses across all years of study data (2011-2021).
Starting with the 2018 schedule, additional guidance was added regarding the minimum acceptable interval between the 3 rd and 4 th DTaP vaccination doses.This guidance clarified that six months is the minimum recommended interval for prospective administration; however, a four-month interval is acceptable in retrospective evaluation.In other words, if it is determined that an already-administered 4 th DTaP dose was at least four months after the prior dose, it does not need to be re-administered.Since this clarification allowed for a less strict minimum interval between the 3 rd and 4 th DTaP vaccine does, we applied it across all years of study data (2011-2021).

Differences in parameters for measuring days undervaccinated across information sources
In eTable1, we provide side-by-side comparisons of parameters identified from three information sources: ACIP schedules and accompanying documentation, CDSi documentation, and previous studies.ACIP and CDSi parameters were similar; however, there were differences in days undervaccinated parameters used for some vaccines in prior studies.
Across information sources, there were multiple differences for the parameters used to calculate days undervaccinated for the Hepatitis B vaccine.The ACIP recommends that medically stable children weighing ≥2,000 grams receive a monovalent HepB vaccination within 24 hours of birth, and all other children should receive the vaccination by age 1 months or at hospital discharge, whichever is first. 8Prior studies have initiated the days undervaccinated count for the first HepB vaccine dose at age 4 days 4 , 32 days 1 , or 93 days. 3,5,6For this study, we set the days undervaccinated count initiation parameter for the dose 1 of the Hepatitis B vaccine to age 31 days, since this is the oldest a child can be in age in days when turning 1 month old, assuming that birth is day 0.
ACIP and CDSi documentation both state that the minimum age of receipt of the 3 rd HepB vaccine dose is 24 weeks; however, Luman et al. expressed this minimum acceptable age as 6 months.As a result, the minimum acceptable age in days for the 3 rd HepB vaccine dose differed across studies, likely due to methodological differences in converting months to days versus weeks to days.For this study, and similar to other studies 5,6 , we set the parameter of the minimum acceptable age of receipt of dose 3 of the Hepatitis B vaccine to 164 days (24 weeks x 7 days=168 days-4-day grace period=164 days).
Additionally, the minimum acceptable interval between HepB vaccine dose 2 and dose 3 differed across studies.The package insert for the combination DTaP-IPV-HepB vaccine (Pediarix®, GlaxoSmithKline) states that vaccine doses should be administered "at intervals of 6-8 weeks, preferably 8 weeks" 9 (38-52 days; 6 weeks x 7 days=42 days-4-day grace period=38 days and 8 weeks x 7 days=56 days-4-day grace period=52 days).Some prior studies calculated the minimum acceptable interval between HepB vaccine dose 2 and 3 to be 38 days to account for the use of the combination DTaP-IPV-HepB vaccines. 3,5,6However, ACIP and CDSi documentation specifies an 8 week interval between doses 2 and 3, and other prior studies used 56 days, minus a 4-day grace period, for this interval. 1,4herefore, for this study, we established this minimum acceptable interval between doses 2 and 3 of the Hepatitis B vaccine to be 52 days between dose 2 and dose 3.
Across previous studies, there was also variation in the parameters applied for the Haemophilus influenzae (Hib) and pneumococcal conjugate (PCV) vaccinations.The final doses of both Hib and pneumococcal vaccine series are recommended by the ACIP at ages 12-15 months, and a child would be considered late on the day they turned age 16 months.Some prior studies set the days undervaccinated count to initiate at age 489 days (approximately age 16 months) for the final doses of both Hib and pneumococcal vaccinations 1,4 ; however, other studies used age 580 days (approximately age 19 months) 3,5,6 to account for the DTaP-IPV-Hib combination vaccine (Pentacel®, Sanofi Pasteur), which the package insert state should be administered at 2,4,6, and 15-18 months. 10ecause pneumococcal, DTaP-containing, and Hib-containing vaccines are often concomitantly administered, these studies also initiated the pneumococcal vaccine series days undervaccinated count at age 19 months. 3,5,6However, since the ACIP recommendations are for the fourth doses to be administered at 12-15 months, we initiated the days undervaccination calculation for the booster vaccination doses of both Hib and PCV at age 489 days for this analysis.
In 2007, the ACIP recommended age for varicella vaccination changed from 12-18 months to 12-15 months. 11Luman and colleagues published their seminal work on vaccination timeliness in 2005 2 , prior to this change.4][5][6] Similar to other previous studies, in this study we began days undervaccinated count calculations at age 489 days.

Parameters for measuring days undervaccinated for the first dose of hepatitis A vaccination
To our knowledge, parameters for calculating days undervaccinated for HepA vaccination have not been previously published.During the study period, the ACIP recommended a two dose Hepatitis A series for children ages 12-23 months (inclusive), with a minimum interval between doses of 6 months.Since we were measuring days undervaccinated for children ages 0-19 months, we only established parameters for the first dose of HepA vaccination.Per ACIP recommendations, the minimum acceptable age of receipt for dose 1 is age 12 months (365 days-4-day grace period= 361 days).Given that a minimum of 6 months is required between the two recommended HepA doses (and allowing for a 4-day grace period), to complete the 2-dose HepA series by the child's second birth day, children must receive the first dose before age 550 days.Therefore, the days undervaccinated count initiation parameter for the first dose of Hepatitis A vaccination was set to age 550 days in this study.

eTable 1 .
Comparisons of Parameters for Measuring Days Undervaccinated for Early Childhood Vaccinations Across Information Sources Parameters for recommended age for routine administration and minimum acceptable age of receipt Recommended
© 2024 Newcomer SR et al.JAMA Network Open.© 2024 Newcomer SR et al.JAMA Network Open.--,Not included in source documentation; ACIP, Advisory Committee on Immunization Practices; CDSi, Clinical Decision Support for Immunization; d, days; DTaP, Diphtheria, tetanus, acellular pertussis; Hib, Haemophilus influenzae type b; HepA, Hepatitis A; HepB, Hepatitis B; IPV, inactivated poliovirus vaccine; MMR, measles, mumps, rubella; mo, months; PCV, pneumococcal conjugate vaccine; RV, rotavirus; VAR, varicella; wk, week a f In 2018, the ACIP published additional guidance regarding the minimum acceptable interval between the 3 rd and 4 th DTaP doses, which clarified that 6 months is the minimum recommended interval for prospective administration; however, a four-month interval is acceptable in retrospective evaluation.g Per ACIP recommendations, the maximum age for RV dose 1 is 14 weeks, 6 days.Children are not recommended to start the series on or after age 15 weeks, 0 days.The maximum age for the final dose is 8 months, 0 days.eAppendix.Methods for Determining Parameters for Measuring Days Undervaccinated

144 n=16,687 n=13,611 n=14,893 n=15,167 n=14,988 n=15,333 n=15,657 n=16,365 n=19,342
Prevalence of Children Who Received All Vaccine Doses On-Time (i.e., Zero Days Undervaccinated), for the Combined 7-Vaccine Series and for Nine Recommended Vaccinations, Among US Children by Age 19 Months, National Immunization Survey-Child 2011-2021 Associations Between Socioeconomic, Demographic, Household and Other Factors and On-Time Receipt of the Combined 7-Vaccine Series, National Immunization Survey-Child 2011-2021 If the child was on any form of Medicaid, alone or in combination with other types of health insurance status, the child was classified as Any Medicaid in the NIS-Child survey.cOtherinsurance includes CHIP, IHS, Military, Tricare, CHAMPUS, or CHAMPVA (alone or in combination with private insurance).dTheseare the race and ethnicity categories provided in the NIS-Child public-use datasets.The non-Hispanic other and multiple races category includes Asian, American Indian, Alaska Native, Native Hawaiian, Pacific Islander, and/or multiple races.Associations Between Socioeconomic, Demographic, Household and Other Factors and On-Time Receipt of the Combined 7-Vaccine Series, Among a Subcohort of US Children Who Completed the Combined 7-Vaccine Series by Age 19 Months, National Immunization Survey-Child 2011-2021 If the child was on any form of Medicaid, alone or in combination with other types of health insurance status, the child was classified as Any Medicaid in the NIS-Child survey.cOtherinsurance includes CHIP, IHS, Military, Tricare, CHAMPUS, or CHAMPVA (alone or in combination with private insurance).dTheseare the race and ethnicity categories provided in the NIS-Child public-use datasets.The non-Hispanic other and multiple races category includes Asian, American Indian, Alaska Native, Native Hawaiian, Pacific Islander, and/or multiple races.eTable8.Associations Between Socioeconomic, Demographic, Household and Other Factors and On-Time Receipt of the Combined 7-Vaccine Series, Including Effect Modification by Poverty Status, National Immunization Survey-Child 2011-2021 These are the race and ethnicity categories provided in the NIS-Child public-use datasets.The non-Hispanic other and multiple races category includes Asian, American Indian, Alaska Native, Native Hawaiian, Pacific Islander, and/or multiple races.cIf the child was on any form of Medicaid, alone or in combination with other types of health insurance status, the child was classified as Any Medicaid in the NIS-Child survey.Associations Between Socioeconomic, Demographic, Household and Other Factors and On-Time Receipt of the Combined 7-Vaccine Series, Including Effect Modification by Health Insurance Status, National Immunization Survey-Child 2011-2021 , adjusted prevalence ratio; CI, confidence interval; NIS-Child, National Immunization Survey-Child; PR, prevalence ratioa Due to missing covariate data, 166,316/179,154 observations were included in multivariable analyses (90.3%, weighted), including 56,607/59,445 children with zero days undervaccinated and complete covariate data (92.4%,weighted).The following variables were missing data: vaccine provider facility type (n=2,943 missing observations, 1.1%, weighted), poverty status (n=6,150 missing observations, 6.0%, weighted), home ownership (n=524 missing observations, 0.4%, weighted), and insurance coverage (n=4,107 missing observations, 2.9%, weighted).b If the child was on any form of Medicaid, alone or in combination with other types of health insurance status, the child was classified as Any Medicaid in the NIS-Child survey.c Other insurance includes CHIP, IHS, Military, Tricare, CHAMPUS, or CHAMPVA (alone or in combination with private insurance).d These are the race and ethnicity categories provided in the NIS-Child public-use datasets.The non-Hispanic other and multiple races category includes Asian, American Indian, Alaska Native, Native Hawaiian, Pacific Islander, and/or multiple races.